Thy belly to swell, and thy thigh to rot - What is meant by these expressions cannot be easily ascertained. לנפל ירך lanpel yarech signifies literally thy thigh to fall. As the thigh, feet, etc., were used among the Hebrews delicately to express the parts which nature conceals, (see Genesis 46:26), the expression here is probably to be understood in this sense; and the falling down of the thigh here must mean something similar to the prolapsus uteri, or falling down of the womb, which might be a natural effect of the preternatural distension of the abdomen. In 1-Corinthians 11:29, St. Paul seems to allude to the case of the guilty woman drinking the bitter cursed waters that caused her destruction: He who eateth and drinketh unworthily, eateth and drinketh damnation (κριμα, condemnation or judgment) to himself; and there is probably a reference to the same thing in Psalm 109:18, and in Daniel 9:11.
And the woman shall say, Amen, amen - This is the first place where this word occurs in the common form of a concluding wish in prayer. The root אמן aman signifies to be steady, true, permanent. And in prayer it signifies let it be so - make it steady - let it be ratified. Some have supposed that it is composed of the initial letters of אדני מלך נאמן Adonai Melech Neeman, My Lord the faithful King, but this derivation is both far-fetched and unnecessary.

And this water that causeth the curse shall go into thy bowels, to make [thy] belly to swell, and [thy] thigh to rot: And the woman shall say, (l) Amen, amen.
(l) That is, may it be as you wished, as in (Psalm 41:13; Deuteronomy 27:15).

And this water that causeth the curse,.... Upon the drinking of which the curse follows, if guilty:
shall go into thy bowels; and there operate and produce the above effects, which are repeated again to inject terror:
to make thy belly to swell, and thy thigh to rot; here ends the form of the oath, which begins Numbers 5:19,
and the woman shall say, amen, amen; so be it; let it be as pronounced, if I am guilty; which, as Aben Ezra observes, is repeated for the sake of confirmation; though the Jewish writers commonly understand it as respecting various things, the oath and the curse, the thing charged with, and the persons suspected of (x).
(x) Misn. ib. sect. 5. Targum Jonah. & Jerus. & Jarchi in loc.

the woman shall say, Amen, Amen--The Israelites were accustomed, instead of formally repeating the words of an oath merely to say, "Amen," a "so be it" to the imprecations it contained. The reduplication of the word was designed as an evidence of the woman's innocence, and a willingness that God would do to her according to her desert.

Amen, amen - That is, so let it be if I be guilty. The word is doubled by her as an evidence of her innocency, and ardent desire that God would deal with her according to her desert.